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What Is Lvdt? What Is The Principle Of Operation OF Lvdt

What Is Lvdt? What Is The Principle Of Operation OF Lvdt

September 04 , 2023 0

LVDT (Linear Variable Differential Transformer) stands for Linear Variable Differential Transformer and belongs to the category of linear displacement transducers. The principle of operation is simply an iron core movable transformer. It consists of primary coil, secondary coil, iron core, coil skeleton, shell and other components. During LVDT operation, the movement of the iron core must not exceed the linear range of the coil. Otherwise, non-linear values will be generated, so all LVDTs have a linear range.

What is an LVDT?

LVDT stands for Linear Variable Differential Transformer. It is a common type of electromechanical transducer that converts the linear motion of an object to which it is mechanically coupled into a corresponding electrical signal.LVDT linear displacement transducers are readily available to measure a wide variety of movements as small as a millionth of an inch to a few inches, but are also capable of measuring positions as large as ±30 inches (±0.762 meters). Figure 1 shows the components of a typical LVDT. The internal structure of the transformer consists of a primary winding and a pair of secondary windings wound in the same manner, with the two secondary windings symmetrically distributed on either side of the primary winding. The coils are wound on a thermally stable, single-piece, hollow glass reinforced polymer with a moisture barrier, wrapped in a magnetic shield with high permeability, and then secured in a cylindrical stainless steel jacket. This coil fitting is typically the stationary element of a displacement transducer.

The primary winding is shown in the center of the LVDT. The two secondary windings are either symmetrically wound on either side of the primary winding (as shown in the diagram for "short travel" LVDTs), or on top of the primary winding (for "long travel" LVDTs). The two secondary windings are usually connected in "reverse series" (differential).

The moving element of the LVDT is a free-standing tubular armature of permeable magnetic material. This is called a fiber core and is free to move axially within the hollow bore of the coil and is mechanically coupled to the object at the desired measurement position. The bore is usually large enough to provide a large radial clearance between the fiber core and the bore so that no physical contact is made between it and the coil. In operation, the primary windings of the LVDT are energized by an alternating current of suitable amplitude and frequency, a process known as primary excitation.The electrical output signal of the LVDT is a differential AC voltage between the two secondary windings, which varies with the axial position of the core within the LVDT coil. Typically, this AC output voltage is converted to a more user-friendly high level DC voltage or current by appropriate electronic circuitry.

How does LVDT work?

The following pictures show what happens when the core of an LVDT is in different axial positions. The primary winding P of an LVDT is energized by a constant amplitude AC supply. The resulting magnetic flux is coupled from the core to the adjacent secondary windings S1 and S2. If the core is located in the middle of S1 and S2, an equal amount of flux is coupled to each of the secondary windings, so that windings S1 and S2 each contain equal amounts of E1 and E2. At this reference intermediate core position (called the zero point), the differential voltage output (E1 - E2) is essentially zero. As shown in Figure 2, if the core is moved so that it is less distant from S1 than it is from S2, the magnetic flux coupled to S1 increases and the magnetic flux coupled to S2 decreases, resulting in an increase in the induced voltage, E1, and a decrease in E2, resulting in a differential voltage (E1 - E2). Conversely, if the core is moved closer to S2, the flux coupled to S2 increases and the flux coupled to S1 decreases, so E2 increases and E1 decreases, resulting in a differential voltage (E2 - E1).

Shows what happens when the core of an LVDT is in a different axial position.

Picture 3 below shows how the magnitude of the differential output voltage EOUT varies with core position. The value of EOUT for maximum core displacement from zero depends on the amplitude of the primary excitation voltage and the sensitivity factor of the particular LVDT, but is typically a few volts RMS. The phase angle of this AC output voltage, EOUT (using the primary excitation voltage as a reference) remains constant until the center of the core passes through zero, at which point the phase angle suddenly changes by 180 degrees, as shown in Figure 3B. This 180-degree phase shift can be used to determine the direction of the core away from the zero point through the appropriate circuitry. This is shown in FIG. 3C, where the polarity of the output signal indicates the position of the core in relation to the zero point. The figure also shows that the output of the LVDT is very linear over its specified core movement range, but that the transducer can be used over a wider range, where the output linearity decreases.

The output characteristics of an LVDT vary with the position of the fiber core. The full range output is a large signal (typically one volt or greater) and usually does not require amplification. Note that the LVDT will continue to operate at more than 100% full range, but with reduced linearity.

Why use LVDT displacement sensors?

LVDT displacement transducers have certain characteristics and advantages due to the basic physical principles of their operation or the materials and technologies used in their construction.

Frictionless Operation

One of the most important characteristics of LVDTs is their frictionless operation. Under normal operation, there is no mechanical contact, such as friction, drag, or other factors that could cause friction between the core and coil assembly structure of an LVDT. This feature is particularly useful in materials testing, vibration displacement measurements, and high-resolution dimensional measurement systems.

Infinite Resolution

Because LVDTs operate on the electromagnetic coupling principle using a frictionless structure, they are able to measure extremely small changes in core position. This infinite resolution capability is limited only by the resolution of the LVDT signal amplifier and the number of bits in the output display. As a result, the LVDT has excellent repeatability.

Unlimited Mechanical Life

Normally there is no contact between the core and coil assembly structure of an LVDT, so there is no friction or wear between any of the components, which means that LVDTs are characterized by unlimited mechanical life. This feature is particularly important in applications where high reliability is required, such as aircraft, satellites, space vehicles and nuclear power installations. It is also in high demand for many industrial process control and factory automation systems.

No damage due to out-of-measurement range

Most LVDTs are not closed at both ends of the bore. If the rated measurement travel is exceeded due to inadvertent operation, the core passes completely through the structure of the sensor's coil assembly without causing any damage. This non-destructive feature makes the LVDT an ideal transducer for use in material damage test equipment such as tensiometers that are attached to a test component to test its tension, see the HSTA750 series.

Single Axis Sensing

The LVDT responds only to core movement along the axis of the coil, and does not normally sense movement of the core transverse to the cross-axis or the radial position of the core. Therefore, for applications where the LVDT is mounted out of alignment or floating, and where the movement of the LVDT is not precisely in a straight line, the LVDT will still function without any counterproductive effects.

Separable Coils and Cores

Since there is only a magnetic coupling interaction between the core and the coil assembly of the LVDT, a non-magnetic tube can be inserted between the core and the bore wall to isolate the core from the coil assembly. In this way, the pressurized fluid can be encapsulated in the isolated tube, so that the core will move freely when the coil assembly is pressurized. This feature is often utilized in proportional or servo valves in hydraulic systems as LVDTs for return of spool position, refer to HSIR750 series.Related sensor devices can be found in

Excellent environmental resistance

LVDTs are very rugged sensors due to the materials used and the technology of the assembly structure, and can be used in a wide variety of operating environments. The coil winding is encapsulated with epoxy resin and placed inside a stainless steel tube, which provides excellent resistance to moisture and humidity, as well as vibration and shock. In addition, the internal high permeability magnetic shielding minimizes the influence of external AC magnetic field effects.

The housing and core are made of corrosion-resistant metal, and the housing also provides enhanced isolation from magnetic fields. In applications where the sensor must be exposed to flammable or corrosive vapors and liquids, or to pressurized fluids, a variety of fusion bonding processes can be utilized to completely seal the housing and coil assembly structure.

Normally LVDTs are suitable for a wide range of operating temperatures but, if required, they can be customized for use in cryogenic environments. Alternatively, special materials can be used for applications such as nuclear reactors that require high temperature and radiation resistance.

Zero Repeatability

The zero position of the LVDT is inherently stable and highly repeatable, even beyond its operating temperature range. This feature makes the LVDT an excellent zero position sensor for closed loop control systems and high performance servo balancing instruments.

Fast Dynamic Response

Under normal operating conditions, this frictionless feature allows the LVDT to respond very quickly to changes in core position, and the dynamic response of the LVDT itself is limited only by the inertial effects of the tiny mass of the core. In general, the speed of response of an LVDT inductive system depends on the characteristics of the amplifier.

Absolute Output

The LVDT is an absolute output component, as opposed to an incremental output component. This means that the position data from the LVDT does not disappear even when the power supply is switched off. When the measurement system is restarted, the output value of the LVDT will be the same as before the power was turned off.